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=3x/4+(x+1/2)=-11/4
MTC:4
=3x/4+[(2(x+1)/4=-11/4
=3x+2x+2=-11
=5x+2=-11
=5x=-13
=x=-13/5
(=) 3/4x + 1/2x +1/2 = -11/4 (=) 5/4x = -1/2 - 11/4 (=) 5/4x = -13/4 (=) x = -13/5
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i/
\(=\dfrac{2-1}{1.2}+\dfrac{3-2}{2.3}+\dfrac{4-3}{3.4}+...+\dfrac{50-49}{49.50}=\)
\(=1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{49}-\dfrac{1}{50}=\)
\(=1-\dfrac{1}{50}=\dfrac{49}{50}\)
k/
\(=\dfrac{1}{3}\left(1+\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}+\dfrac{1}{32}\right)=\)
Đặt biểu thức trong mhoawcj đơn là k ta có
\(2k=2+1+\dfrac{1}{2}+\dfrac{1}{4}+\dfrac{1}{8}+\dfrac{1}{16}\)
\(\Rightarrow k=2k-k=2-\dfrac{1}{32}=\dfrac{63}{32}\)
\(\Rightarrow\dfrac{1}{3}.k=\dfrac{1}{3}.\dfrac{63}{32}=\dfrac{21}{32}\)
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ta có: a.3 = 3 + 32 + 33 + ... +32009 a3 - a = (3 + 32 +33 + ... + 32009) - (1 + 3 + 32 + ... + 32008) a2 = 32009 - 1 a = 32009 - 1 / 2
18.C 19.A maybe =))